1. Field of the Invention
The present invention relates to a process for treating and recovering a pickling waste liquid for stainless steel, particularly nitric-hydrofluoric acid, nitric acid, sulfuric acid or hydrochloric acid waste liquid containing ions of Fe, Cr and Ni used in the pickling of stainless steel sheet.
2. Description of the Prior Art
At the pickling step in the stainless steel making, solution of a mineral acid such as nitric-hydrofluoric acid, nitric acid, sulfuric acid, hydrochloric acid or the like is usually used, from which various pickling waste liquids containing ions of Fe, Cr and Ni are produced.
Heretofore, the treatment of nitric-hydrofluoric acid or nitric acid waste liquid has exclusively been performed through an alkali neutralizing process. In this case, sludges such as metal hydroxides and metal fluorides are produced in a large amount, and nitrogen content (NO.sub.3.sup.-) is discharged which is a cause of eutrophication in the public water area. Accordingly, it has been demanded to develop a technique for recovering nitric-hydrofluoric acid and metals from the waste liquid.
In this connection, a solvent extraction process is noted as a recovering technique. This technique is disclosed in Japanese Patent Application Publication No. 56-42,674, wherein Fe.sup.3+ is first selectively extracted from the waste liquid of nitric-hydrofluoric acid with an organic solvent containing alkyl phosphoric acid. The thus extracted Fe.sup.3+ is stripped by a process using an aqueous solution of fluoride series as described in Japanese Patent laid-open No. 57-42,545 and recovered as an iron oxide or a metallic iron by a process as described in Japanese Patent laid-open No. 58-15,039 or Japanese Patent Application Publication No. 58-12,323. On the other hand, at least an equivalent amount of sulfuric acid or hydrochloric acid is added to metal ions contained in the waste liquid after the extraction of Fe, and then the nitric-hydrofluoric acid is recovered with an organic solvent containing a neutral phosphate ester. According to this process, the nitric-hydrofluoric acid can be reused in a high yield, but the problem still remains in the treatment for ions of Ni and Cr included in the residual liquid after the extraction of nitric-hydrofluoric acid. For example, if it is intended to recover Ni and Cr by solvent extraction process or the like, the recovering step becomes complicated.
And also, the treatment for sulfuric acid or hydrochloric acid waste liquid has hitherto been carried out by the alkali neutralizing process. In this case, however, the cost in the oxidation of Fe.sup.2+ becomes expensive and a large amount of hydroxide sludge is produced and is difficult to effectively use. Accordingly, aiming at the presence of Fe.sup.2+ in the sulfuric acid or hydrochloric acid waste liquid, there has been proposed such a technic that this metal ion is recovered together with ions of Cr and Ni as ferromagnetic oxide by a ferrite formation process as shown in Japanses Patent Application Publication No. 51-22,307. The thus recovered ferrite is useful for a radio wave absorber and a vibration damper and has a high additional value. However, this ferrite formation process is inapplicable to the treatment of nitric acid or nitric-hydrofluoric acid and produces a concentrated solution of sodium sulfate when being applied to the treatment of the sulfuric acid waste liquid. In the latter case, there is a drawback that when the resulting waste water is led to a treating equipment for usual lime neutralization, a gypsum is produced, resulting in the cause of troubles.
As mentioned above, the solvent extraction process can treat the nitric acid or nitric-hydrofluoric acid waste liquid, while the ferrite formation process can treat the sulfuric acid or hydrochloric acid waste liquid. However, it is customary to simultaneously use the nitric acid or nitric-hydrofluoric acid and the sulfuric acid or hydrochloric acid in the pickling step for stainless steel, so that it is desired to treat these waste liquids synthetically.